Dielectric heating mold and method of forming a wooden mold member



Aug. 2, 1960 l. J.

DIELECTRIC HEATING MOLD AND METHOD on BARSY FORMING A WOODEN MOLD MEMBER Filed Jan. 25, 1957 N- h 0 m ,v m

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INVENTOR IMRE J. BARSY ATTORNEY United States Patent" `DIELECTRIC HEATING MOLD AND METHOD VF 'FORMING A WOODEN MOLD MEMBER Imre J. Barsy, Lancaster Township, Lancaster County, Pa., assignor to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Filed Jan. 25, 1957, Ser. No. 636,289

'12 Claims. (Cl. 18-47) of cork' composition blocks, for instance. The side walls i of the Scott mold are made of hard wood impregnated with ceresin wax.

Gard Patent 2,707,801 also discloses a similar mold structure in which the conventional wood mold as disclosed in the Scott patent is provided on its inner wall surfaces with liners which possess special characteristics permitting the mold to be used for dielectric heating where alternate runs of different materials, such as resin-bound cork tile and glue-bound industrial cork compositions, are molded.

As these prior patents have indicated, `one of the problems involved in the molding of cork compositions and other products in molds with wooden walls has been to provide a molding surface from which the molded mass may be released after completion of each molding cycle.

` An object of the present invention, therefore, is to pro? vide a mold-with walls of wood for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the wooden mold 'walls are both dielectrically heated simultaneously, which mold walls will have essentially permanent release characteristics, permitting ready extraction of the molded product from the mold upon completion of each molding cycle.

Another object of the invention is to provide a mold ot" the foregoing type xin which the wall members of wood are essentially completely impregnated throughout with a resin cured in situ within the pores of the wood, minirnizing the absorption of moisture within the mold walls even under unusually severe service conditions, while at the same time providing essentially permanent release characteristics. y

An additional object ofthe invention is to provide a mold of the' type mentioned in which the resinV impregnant is so fixed within the body of the wood as to be essen- -ti'ally proof against bleeding or expulsion upon dielectric rials such as cork compositions.

Another object of the invention is to provide a method t ICC of fabricating wooden mold wallswhich will provide-essentially complete direct replacement of all moisture within the wood by `a body of resin which is then polymerized or otherwisecured directly within the cellular body of the wood without objectionable deterioration of the strength and other physical properties of the wood.

Other objects oftheinvention `will be clear from the following description of an embodiment of the invention.

In the attached drawing: i i

Figure l is a diagrammatic view, partially in section, illustrating a typical mold structureV embodying the in vention; and

Figure 2 is an infrared spectrophotometer curve for a silicone resin impregnating composition. l

The mold structure A typical mold structure is shown in Figure l. `This type of mold is disclosed in'Lockwood Patent 2,625,740. It is .typical of the type of mold to which the present .invention is applicable. It includes end wall panels 2 and 3 and `side wall panels 4 and 5 made of impregnated wood. 'Structural members of metal 6 and 7 are provided which receive the wooden wall members. .End plates 8 and 9 form the top and bottom walls of the mold and constitute electrodes which are coupled to a high frequency source, as diagrammatically indicated in Figure 1. l The wooden mold wall panels specific physical characteristics which will be enumerated below and which serves when so combined withinthe body of the wood to provide not only essentially permanent release characteristics for the mold wall but also other Vdesirable properties such as durable water repellency, with the internal void or cellular structure ofthe lwood and the exposed surfaces being essentially sealed against water in liquid form. This insures against a continuous film of moisture along a path on the surfaceor through` the' cross section of the panelan important characteristic where the mold wall is heated dielectrically along with the charge in the mold.

The preferred silicone resin is one which is air' drying rand cures or sets by polymerization and hydrolysis.-A It has a resin solids content of 90% and has a viscosity at 25 C. in the range of 10-100 centipoises (Saybolt Uni- `versal Viscometer). Its specific gravity at 25 C. is I1.05

to l;l5, and its color is light straw. It is soluble in xylene l'which is used as a solvent to reduce its viscosity within .permissible limits for proper impregnation. The mate- .'rial when subjected to infrared analysis has a curve which is illustrated in the attached drawing as Figure 2.

The preferred silicone resin is a mixture of a partially hydrolyzed methyl substituted polys'iloxane containing 2d-50% by weight of alkoxy and the reaction product of -a silica hydrosol and chlorinated methyl silane. Vpreparation of a resin of this sort is disclosed in Bass The Patents 2,706,723 and 2,706,724 to which reference is madefor further details of the resin and its preparation. Ordinaryorgano polysiloxanes and straight chain silicones -such as Dow Corning Corporations Silaneal or General Electric Companys Dri-Wall will not provide equivalent results. The mixture of the two components mentioned above, i.e. a mixture of a partially hydrolyzed methyl substituted polysiloxane containing -5 0% by weight of alkoxy and th'e reaction product of a silica hydrosol and chlorinated methyl silane, when impregnated into a body K 2.56 Tan a a Y .052 LossI factor .134

The. valuesy were determined ata frequency of. 13.5 xrlegacycles, at.. 70 F., with the wood grain perpendicular to the thickness of the blank under examination.

Other silicone resins having similar characteristics may be substituted, provided. they meet theA following specications: l

( l) At room temperature, C., the resin impregnatcomposition should have a viscosity lin the range of 104ml: centipniscs with a resin `solids coment of. 75% or more-l, Thelower the viscosity of the resin composition, the,v better the penetration and the higher the. percentage of absorption. This. is an important consideration, since maple ,and other suitable hard woods for mold walls are rather dense, andfor thick panels thorough impregnation is. difficult. to achieve even under` ideall conditions'. Y

' Q2); The resin composition musthave a maximum timetemperature reaction factor which is the equivalent of 120 minutes at 320 F. A resin which will cure. at a moderate temperature in the order of 230 F. to 240 F. is preferred. This maximum time-temperature reaction .factor is essential to insure that theresin may be fully .cured under temperature and time conditions which will not deleteriously affect the wood. As is well recognized, wood will-withstandextremely high temperatures (320 F,...i`orV example) only for relatively short periods of time without deterioration. Wood is seriously deteriorated if suhiected to relatively lower temperatures only if they 312e. appliedv over extremely long periods of time. Thus, Vthe time-temperature reaction Vfactor plays an important partfin achieving success, for the resin within the Wooden mold wallsvmust be cured without substantial sacrice ofthe physical strength and other characteristics of the wood.

(3,) The minimum silicone content of the resin composition. should be 75%, the balance being the solvent carrier and what may be termed modifying agents. Optimum range of silicone solids resin content is 7590%. ['Ifheimpregnating resin should contain no catalyst. The .surface coating resin which will be discussed below may contain `up to 2%, to 3% of catalyst. Extensive experimentation has shown that even relatvely minor amounts of other materials, such as phenol-formaldehyde conden- .sation .products which are sometimes compounded with vsiliconefresins,4 substantially reduce or actually destroy the release properties of the finished mold panel. Minor amounts of. some modifying agents can ber tolerated, but ifthey are `used it is preferred' to keep them small. Conveutional siliconeoils and silicone waxes which have been recommended as release agents for coating mold surfaces have not been found to operate effectively vas impregnants for wooden mold vmembers for dielectric heating to obltain permanent releasey characteristics and the other phy- `Vsicall properties which a Wooden mold Wall must possess. Permanent release characteristics have never been attained with-suchcompositions, and with some o f them the bond 'between the. dielectrical'ly heated molded materialland the surface of the mold wall has been greater than the strength of the material itself, resulting in actual rupture ofthe product upon attempted extraction of it from the mold upon completion of the molding cycle.

(4) They should provide in the iinished panel a loss factor in the range of .l to .4.

The fabrication ofthe mold wall panels The following description is directed to the formation of atypical mold Wall panel for a mold such as shown in Figure l and useful in the molding of cork compositions, such as cork tile mats or cork composition blocks for gaskets, friction elements, or the like, employing dielectric heating to cure the binder used to join the cork particles together.

Rectangular pieces or blanks of' hard. maple ksapwood (sugar maple-% sapwood) are cut from seasoned, kiln-dried planks, and only flawless blanks acceptable. for electrical uses are selected Vfrom the cut pieces. In the fabrication of a mold for dielectric heating of a mass of cork compositionl about 50 by 28 by 9', the rectangular blanks may be approximately 17j/s" by 95/6" by 2%" thick.

These rectangular blanks of wood are heated dielectrically as disclosed in Gard lPatent.2,631,109. The temperature elevated to about .270 to 280 F., although temperatures' in the range of 250 to 300 F. may he .used APreferred. practice is to dielectrically heat the blanks .on the. same day vthat the wood is cnt from the planks. `The dielectric heating rateused should'be moderate, involving ll/z to. 2 hours, for example, for twenty blanks. of the sizel mentioned above to. attain a Vtemperature of about 280 F. This initial dielectric heating serves to4v drive.- moisture. fromwithin the pores of the wood,

Vand this result. is achieved without, serious strains being developed inthe wood.

Upon completion ofthe dielectric heating, the blanks are placed in a forced convection oven at a temperature of about F. for about 7 tor 10 days, This serves to complete the removal of substantially all moisture from the wood and also to release all stresses in the wood so that any warpage can be cut away in the nal fabrication ofi-the mold wall from the rectangular blanks. It also serves toinduce an antishrink eiiciency in the pieces so that, upon subsequent heating during operation of the mold, Aopenings between thek panels constituting the. mold walls will not occur or. at least are greatly mimmizd, and objectionable` checks. and cracks in the rblanks will .be eliminated.. At, the end. of theoven drying OperQn,

the blanks are essentially bone dry.

The blanks` are then cut to4 the iinal nshed panel shape and size except that. the thickness of the panels is made about la oversize to allow for iinal facing `during or after installation of the panels into the metal mold supporting members, as shown in the drawing. It is preferred to; `so Vfabricate the blanks into panels to avoid impregnation is. effected; for this` will insure that the interior mold surfaces will be fully impregnated. With the process of the present invention, impregnation will be effected throughout the whole thickness of the board;

although,l asrnentioned. above, there will be a greater concentration of the impregnating .ComPOSOD adjacent to the outer surfaces of the pieces.

The shaped panels are inspected to eliminate those with electrically or mechanically objectionable aws, and the Slted panels. are, heateda second time bythe diobjectionable.

electric eect. This time they are elevated to a temperature above 230 F., preferably in theorder of 290 to'300' F. It is vpreferred to maintain the temperature below about 320 F. to avoid damage to the wood. Av moderate heating rate is employed. For example, a batch of twenty panels may be raised to a temperature of about 300 F. in approximately one hour. There is a slight moisture pickup during the fabrication operations, and this is removed by the second dielectric heating. The Wood is also elevated substantially in temperature, and impregnation which follows is enhanced by reason of the partial vacuum created within the body of the wood -as its temperature falls.

The heated panels are immersed while hot, immediately upon completion of the heating cycle, in a thermally insulated tank of resin composition. The resin may be at room temperature. The panels are kept immersed in the resin for aI period of 72 hours and are then removed and permitted to drain.

While the resin is at room temperature at the start of the impregnation cycle, after Vthe 20 panels have been immersed in the tank, the temperature of the uid will have been raised by the heated panels to about 140 to 160 F. The thermally insulated tank and the mass of wood involved assures gradual cooling, and room temperature is reached in 60 to 72 hours. This gradual cooling of the wood and resin during the immersion phase of the process is also a factor in attaining the desired degrec of impregnation. 4The impregnated panels are suspended from a rack with good ventilation and are air-dried for a minimum of 204 days. A longer air drying period is not at all harmful.l This air drying step is important in getting rid of solvent at the surface, Vat least, of the panels and also for starting polymerization or curing of the resin. This minimizes bleeding of the resin when nal curing at elevated temperatures is effected. The air drying may be at room temperature as noted, in the order of 70 to 90 F.

The panels are then suspended in a` well ventilated oven and subjected to the following time-temperature curing schedule:

F. 2 to 21/2 days 125 2 days 150 days 175 l day 200 3 days 230 o'd,'to incorporate as much as'50% of the weight ofthe lwood of resin composition.

Maple panels having 25% of the resin by weight impregnated therein have been found to be satisfactory.

By gradually increasing the temperature over a period lof ten days in the curing cycle before attaining the final curing temperature of 230 F., bleeding of the Aresin is reduced to a bare minimum and an excellent cure is attained. Loss of a few percent of the total weight ofthe resin absorbed by the panels and carried on the surface of the panels may occur through exudation and loss of 4solvent in the process, but this is insignificant.

It is preferred, of course, to cool the blanks Ato room temperature gradually so as to avoid severe shocks on the wood. Also, rapid cooling might result in undesir able cracking or checking of the surface whichI would be It isdesirable during resin curing to heat the panels to a temperature in the order of the tempera- Ature to` be employed in production molding using the 4Wooden mold, generally in the range between 200""F.

spiega-es and 240 F. 'Wherecork' compositions are to be pr-I d uced at a mold wall temperature of about 230 F. as ra maximum, it is desirable to cure the resin in the panels at that temperature since by so doing it will be safe to go to that temperature in actual molding of products, with'- out any fear of loss of impregnant or exudation of the impregnant to the surface of the mold wall where its presence might be objectionable. 'Ihe resin is bonded rmly to the wood but provides a permanent release surface for compositions to be molded.

After the panels are installed in the metal mold membersand properly face finished, the exposed surfaces of the panels are preferably coated either by brushing or ployed except that a catalyst is added to insure proper' cure of the resin at room temperatures and to accelerate air drying. Curing may be accomplished at room temperature in about twelves hours, but a longer periodwill do no harm. The mold is then placed in a dry, clean oven; and the temperature is raised gradually, over 1 to 2 hours, to about 220 F. The coated surfaces are main'- tained at a temperature of 220 to 230 F. for about 8 hours. A longer period at this temperature is not necessary, butit is not harmful. The mold is now ready for service.

I claim:

1. In a mold for confining a charge of dielectric material during processing by dielectric heating in which the charge of material and the mold are both dielectrically heated simultaneously, the combination of a mold side wall formed of hard wood from which essentially al1 moisture has been removed and has been replaced directly Within the cellular body thereof by a body of resin which consists essentially ofthe hard, glasslike heat reaction product of a potentially reactive silicone resin-forming composition comprising a mixture of a partially hydrolyzed methyl substitutedpolysiloxane containing 20- 50% by weight of alkoxy and the reaction product-oa silica hydrosol and chlorinated methyl silane, having a minimum resin solids content of a maximum timetemperature reaction factor the equivalent of 120 minutes at '320 F., and an infrared spectrophotometer curve substantially as shown in Figure 2 cured directly Within the cellular body at a minimum nal curing temperature o about 200 F.

heated simultaneously, the combination of a mold side Wall formed of hard wood from which essentially all moisture has been removed and has been replaced directly within the cellular body thereof by a body of resin which consists essentially of the hard, glasslike heatl reaction product of a potentially reactive silicone resin-forming composition comprising a mixture of a partially `hydrolyzed methyl substituted polysiloxane containing 20- 50% by weight of alkoxy and the reaction product of a silica hydrosol and chlorinated methyl silane, having -a minimum resin solids content of 75%, and a maximum timetemperature reaction factor the equivalent of 120 minutes at 320 F. cured directly within the cellular body 'at a minimum final lcuring temperature of` about 200 F.

resinous composition as the impregnant.

5. A mold in accordance with claim 2in which the vwood is maple sapwood, in which the resin composition has a minimum solids content of about a viscosity tof l0-100 centipoises and is cured at a maximum temperature of about- 240 F., and in which the cured impregnaue constitutesa minimum of about 25% of the weight'of-the wood Y 6. In a method of forming a wooden mold member for a mold for confining a charge :of dielectric material during processing in which the-charge ot material and the mold are both dielectrcally heated simultaneously, the steps comprising forming a panel of hard wood essenn tially free of moisture, impregnating said panel throughout the entire body thereofA with an air drying potentially reactive silicone resineforming composition having .a minimum resin solids content of 75% and comprising a mixe ture ofA a partially. hydrolyzed methyl substituted poly- ,SilOXane containing Y20-50% by weight of alkoxy and the reaction product of a silica hydrosol and chlorinated methyl silane, said resin having a maximum timetem perature reaction factor the equivalent of 120 minutes at 320 F., airV drying said impregnated panel to set the resin against excessive bleeding from Within the cellular body of said panel, and thereafter heating said impregnated panel at a minimum final cure temperature of about 200 F. to cure said resin to a hard, glasslike condition directly Within the cellular body of said panel. A 7. In a method of forming a Wooden mold member for a mold for confining a charge of dielectric material during processing in which the charge of material and the mold are lboth dielectrically heated simultaneously, the steps comprising forming a panel of hard Wood essentially free of moisture, impregnating said panel throughout the entire body thereof with a minimum or" about .25% of the Weight of the Wood of an air drying potentially reactive silicone Vresin-forming composition having a minimum resin solids content of 75% and comprising a mix ,ture of a partially hydrolyzed methyl substituted polysiloxane containing 20-50% by Weight of alkoxy and the reaction Vproduct of a silica hydrosol and chlorinated methyl silane, said resin having a. maximum time-temperature reaction factor the equivalent of 120 minutes at 320 F., air drying said impregnated panel to set the resin against excessive bleeding from within the cellular body of said panehand thereafter heating said impregnated panel at a cure temperature of 200V F. to 240 F. to cure said resin to a hard, glasslike condition directly Within the cellular body of said panel.

*8, In a method of forming a Wooden moldl member `forV a .mold for confining a charge of dielectric material during processing in which the charge of material and the mold are both dielectrically heated simultaneously, the steps comprising forming a panel o hard woodV essentivally free of moisture, impregnating said panel throughout the entire body thereof with an air drying potentially reactive silicone resin-forming composition having a minimum resin solids content of 75% and comprising a miX- tureof a partially hydrolyzed methyl substituted polysiloxane containing 20-50% by weight of alkoxy and the reaction product of a silica hydrosol and chlorinated methyl silane, said resin having a maximum time-temperature reaction factor the equivalent of 120 minutes at 320 F., air drying said impregnated panel to set the resin against excessive bleeding from Within the cellular body o said panel, and thereafter heating said impregnated panel at a cure temperature above about 200 F. and below about 240 F. to cure said resin to a hard, glasslike condition directly within the cellular body of said panel.

- 9. In a method of forming a wooden moldmember for a mold for confining a charge of dielectric material durin g processing in which the charge of material and the mold are both dielectrically heated simultaneously, the steps comprising forming a panel of hard wood essentially free of moisture, heating said panel dielectrically to a temperature above 230 F., immersing said heated panel in a bath of an air drying potentially reactive silicone resin-forming composition having a minimum resin Vsolids, content of 75% and comprising a-'mixture of a partially hydrolyzed methyl ysubstituted polysiloxane containing `2*050% by weight o alkoxy ,andV t,he reaction product ofa silica hydrosol. and chlorinated methyl silane, said resin having a maximumYtime-temperature reaction factor the .equivalent ofV 120 minutes at 320! ZF., permitting said panel to remain in said siliconeresin composition until the wood is impregnated throughout with said resin composition, air drying said impregnated panel, and thereafter heating said impregnatedV panel at a minimum nal cure temperature of about 200 F. to cure said resin to .a hard, glasslike condition directly within the cellular body of said panel.

l0. In aV method of forming a wooden mold member for a mold for conning a charge of dielectric material during processing in which the charge of material andthe mold are both dielcctrically heated simultaneously, the stepscomprising forminga panel of hard Wood essentially freer of moisture, impregnating said panel throughout the entire body thereof with an air drying potentially reactive silicone resin-forming composition comprising a mixture of a partially hydrolyzed methyl substituted polysiloxane containing 2li-50% by weight of alkoxy and the reaction product of a silica hydrosol and chlorinated methyl silane., having an infrared spectrophotometer curve substantially as shown in Figure .2, said resin having a maximum time-temperature reaction factor the equivalent of 120 minutes at 320 F., air drying said impregnated panel to set theresin against excessive bleeding from within the cellular body .of said panel, and thereafter heating said impregnated panel at a minimum linal cure temperature of about 200 F. to cure said resin to a hard, glasslike condition` directly within the cellular body of said panel'.

ll; In a method of forming a Wooden mold member for arnold for contining a charge of dielectric material Vduring processing inV which thecharge of material and the mold: are both dielectrically heated simultaneously, the steps comprising forming a panel of maple `sapwoodre moving moisture from said panel until the same is essen-v tially bone dry, mpregnating said panel with an Yair dry.- ing potentially reactive silicone resin-forming composi tion, said resin composition having a minimum resin solids contentof %Y and a maximum time-temperature `reaction factor` `the equivalent ofV minutes at 320 F., and comprising a mixture of a partially` hydrolyzed methyl substituted polysiloxane containing 20-50% by Weightof alkoxyY and the reaction product of a silica hydrosol and chlorinated methyl silaue, airdryingsaid impregnated panel at a temperature of 70 to 90F. for a Vminimum of twenty days,V oven curingV saidV panel at a temperature of to 200 F. for a minimum of ten days, finally curing said panel at a temperature of 230 F.

for three days, and thereafter cooling said panel to room temperature whereby said resin is cured to a hard, glasslike condition directly Within the cellular body of said panel and is rendered proof against .exudation therefrom under normal dielectric heating conditions at temperatures up to 230 F.

l2. In a method of forming a wooden mold member for a mold for confining a charge of dielectric material during processing in which the charge of material and the mold'are both dielectrically heatedrsimultaneously, the steps comprising forming a panel of hard wood, freeing said panel of all moisture, to provide a panel in substantially bone dry condition, impregnating said panel throughout the entire body thereof with a potentially reactive silicone resinfforming composition having a minimum resin solids content of 75% and a maximum time-temperature reaction factor the equivalent of 120 minutes at 320 F., and comprising a mixture of a partiallyvhydrolyzed methyl substituted polysiloxane containing 20- 5.0% by weight of allroxy and the reaction product of a Vsilica hydrosol and chlorinated methyl silane, which can V ing said resin at a temperature at least equal to the temperature to'beemployed in dielectricv heating, using said mold, said temperature falling in the range between 200 and 240 F.

References Cited in the le of this patent UNITED STATES PATENTS Maisch Apr. 19, 1938 Decker Nov. 28, 1944 Scott Oct. 24, 19'50 Smith July 29, 1952 Gard Mar. 10, 1953 FOREIGN PATENTS Great Britain Mar. 16, 1955 OTHER REFERENCES Rochow: The Chemistry of the Silicones, 1946, pp.

7() and 71. 

2. IN A MOLD FOR CONFINING A CHARGE OF DIELECTRIC MATERIAL DURING PROCESSING BY DIELECTRIC HEATING IN WHICH THE CHARGE OF MATERIAL AND THE MOLD ARE BOTH DIELECTRICALLY HEATED SIMULTANEOUSLY, THE COMBINATION OF A MOLD SIDE WALL FORMED OF HARD WOOD FROM WHICH ESSENTIALLY ALL MOISTURE HAS BEEN REMOVED AND HAS BEEN REPLACED DIRECTLY WITHIN THE CELLULAR BODY THEREOF BY A BODY OF RESIN WHICH CONSISTS ESSENTIALLY OF THE HARD, GLASSLIKE HEAT REACTION PRODUCT OF A POTENTIALLY REACTIVE SILICONE RESIN-FORMING COMPOSITION COMPRISING A MIXTURE OF A PARTIALLY HYDROLYZED METHYL SUBSTITUTED POLYSILOXANE CONTAINING 2050% BY WEIGHT OF ALKOXY AND THE REACTION PRODUCT OF A SILICA HYDROSOL AND CHLORINATED METHYL SILANE, HAVING A MINIMUM RESIN SOLIDS CONTENT OF 75%, AND A MAXIMUM TIME-TEMPERATURE REACTION FACTOR THE EQUIVALENT OF 120 MINUTES AT 320*F. CURED DIRECTLY WITHIN THE CELLULAR BODY AT A MINIMUM FINAL CURING TEMPERATURE OF ABOUT 200*F. 